U.S. Pat. No. 3,737,202 shows three embodiments of generic rolling bearings. The load bearing balls disposed within a respective row are equal in size to one another and to the balls of other rows, i.e. they all have the same rolling element radius. The first embodiment shows rolling bearings of the double row angular contact ball bearing type. The second and third embodiments show mixed types of angular contact ball bearings and radial deep groove ball bearings.
The first embodiment of the generic ball bearing comprises two rows of balls with an inner bearing ring for the first row of balls, a common bearing ring for the first row and the second row as also at least one outer bearing ring for the second row. The common bearing ring comprises an outer raceway for the balls of the radially inner first continuous row and an inner raceway for the balls of the radially outer second continuous row.
U.S. Pat. No. 3,737,202 describes two groups of the pre-cited embodiments. In the first group, the pitch circles of the first row and the second row have the same pitch circle diameter. Because the rows have identical pitch circles and the balls all have the same diameter, the diameters of the inner osculating circles of the rows are equal to each other and the diameters of the outer osculating circles of the rows are equal to each other.
In the second group of the pre-cited embodiments of double row ball bearings, the first row of balls has a pitch circle whose pitch circle diameter is smaller than the pitch circle diameter of the pitch circle of the second row. Because the balls all have identical diameters but different pitch circles, the diameters of the inner osculating circles of the rows are also different from each other and the diameters of the outer osculating circles of the rows are likewise different from each other.
Osculating circles are the circles around the axis of rotation of the rolling bearing on which the contacts of the balls with the raceways are situated.
The second embodiment of the generic ball bearing of U.S. Pat. No. 3,737,202 comprises three rows of balls, one common bearing ring for the rows as also inner bearing rings and outer bearing rings. These triple row ball bearings can be divided into groups as described below.
The triple row ball bearings of the first group comprise a first row and two second rows of baits as well as a common bearing ring for the first row and the second rows. The balls of the first row are arranged on a pitch circle that is smaller than the pitch circle on which the balls of both the second rows are arranged. The pitch circles of the two second rows are equal in size to each other. The balls of the first row run on an outer raceway of the common bearing ring and on an inner raceway of an inner ring. The common bearing ring further comprises two inner raceways for the two second rows. The ball bearing further comprises two separate outer bearing rings each of which comprises an outer raceway for one of the two rows.
The triple row ball bearings of the second group according to U.S. Pat. No. 3,737,202 comprise two first rows of balls whose pitch circles are of equal size to each other, and one second row of balls as well as a common bearing ring for the rows. The pitch circle diameters of the pitch circles of the first rows are smaller than the diameter of the pitch circle of the one second row. Each of the outer raceways for the two first rows is arranged on the common bearing ring. The inner raceways for the two first rows are arranged on a respective inner bearing ring per row. The common bearing ring further comprises an inner raceway for the balls of the second row. Further, to the balls of the second row is likewise associated an outer raceway on an outer bearing ring.
The third embodiment describes a ball bearing with two first rows and one second row. The balls of the three rows are arranged on axially adjacent pitch circles of identical diameters and comprise a common bearing ring. The common bearing ring comprises two outer raceways for the first rows and one inner raceway for the second row. In addition, an inner bearing ring is associated to each of the first rows and an outer bearing ring is associated to the second row.
According to U.S. Pat. No. 3,737,202, advantages of these embodiments are the total rotational speed of the bearing is reduced to approximately half the total rotational speed on each of the rows so that the durability of the rolling bearing is enhanced. Given the case that friction increases in the rolling contact of one of the rows due to wear or heat, its speed of rotation is reduced to an acceptable level and the rotational speed difference is equalized by the other row. In the case that one row fails completely, the bearing still possesses emergency running abilities in the sense that, although the other bearing continues to rotate at the total rotational speed, the roiling bearing itself is not blocked at once.
DE 103 14 259 A1 shows an example of a double row angular contact ball bearing of a generic type in which the two rows have a common bearing ring and different pitch circles. It is known from DE 103 14 259 A1 that, through the use of a common bearing ring, the total rotational speed of the balls of the rows can be substantially halved so that the diameter-rotational speed-characteristic value can likewise be substantially halved.
This characteristic value is also designated as rotational speed characteristic value in the description of technical basics in the Chapter “Lubrication” of the Catalogue HR1 of Schaeffler K G, issued January 2006, and is the product out of rotational speed and pitch circle diameter and is further dependent on the bearing type. This characteristic value is a value that is taken into consideration for the choice of the lubricant grease depending on load and rotational speed conditions. Depending on the bearing type, the limit values for this characteristic value are situated between 350,000 and 600,000. The value is of importance for the durability of the lubricant grease and therefore, also, for the durability of the rolling bearing.
Among other things, the friction within a rolling bearing also depends on the load, the temperatures, the rotational speed and the viscosity of the lubricant grease. In the textbook “Walziagerpraxis” Brändlein, Eschmann, et al, published by Vereinigte Fachbuchverlage GmbH, 1995 Edition, it can be read in the Chapter “Reibung, Ternperatur and Schmierung” on page 210: “The resistance with which a rolling bearing opposes its rotation is made up of rolling friction, sliding friction and lubricant friction.” To be read further is: “The importance of the friction results from the fact that it determines the heat produced in the bearing and thus influences the temperature of the hearing components and of the lubricant”. On page 213 of the same chapter it is stated: “The lubricant friction in a bearing is made up of the internal friction of the lubricant at the contact points and the splashing and flexing work resulting from superfluous lubricant and higher speeds of rotation. The total lubricant friction depends in the first place on the quantity and viscosity of the lubricant. At lower speeds of rotation, this friction is generally low. However, it increases clearly as a function of the oil viscosity or grease consistency with increasing speed of rotation.”
The rule of thumb for grease-lubricated rolling bearings is therefore: In bearings with higher operational speeds of rotation, use lubricating greases with lower consistencies. Besides this, as a rule, due to the requirements made of them, these lubricating greases are substantially more expensive than lubricating greases that can be used in bearings with lower speeds of rotation. In addition, with increasing speeds of rotation and, therefore, increasing influence of centrifugal force and/or at higher temperatures, it is more difficult to keep the lubricating grease in the rolling contact areas and in the rolling bearing due to the relatively low consistence of the lubricating grease. For this reason, specially configured cages and complex seals are often required in such bearings, and this also makes the rolling bearings more expensive.
In all rolling bearings with curved raceway and rolling element profiles, the radius of the raceway profile is slightly larger than the corresponding radius of the rolling element. This difference of curvature in the axial plane is characterized by the osculation. By osculation is to be understood the groove oversize relative to the rolling element radius. [“Wälzlagerpraxis” Brändlein, Eschmann, et al., Vereinigte Fachbuchverlage GmbH, 1995 Edition]. The invention therefore concerns rolling bearings in which the radius of the raceways in the rolling contact areas is larger than the radius of the rolling elements at these points.